1. Field of the Invention
The present invention relates to an anti-ferroelectric liquid crystal display (LCD) device.
2. Description of the Related Art
An anti-ferroelectric LCD device uses a stability of a molecular orientation of an anti-ferroelectric liquid crystal. A simple-matrix type anti-ferroelectric LCD device has been known.
FIGS. 8 and 9 show a structure of the anti-ferroelectric LCD device. A pair of transparent substrates (glass plates or the like) 1 and 2 are arranged to face each other with a liquid crystal layer being interposed therebetween. A plurality of parallel scan electrodes (transparent electrodes) 3 are formed on one substrate 1, and a plurality of parallel signal electrodes (transparent electrodes) 4 are formed on the other substrate 2 to intersect at right angles with longitudinal axis of the scan electrodes 3.
Alignment films 5 and 6 are provided on electro-deformation surfaces of both substrates 1 and 2. These alignment films 5 and 6 are homogeneous alignment films made of an organic high molecular compound such as polyimide. The film surfaces of the alignment films 5 and 6 are subjected to aligning treatment by rubbing them in a mutually parallel direction.
The substrates 1 and 2 are bonded to each other at their peripheral portions with a frame-shaped sealing member (not shown) being interposed therebetween. An anti-ferroelectric liquid crystal 7 is sealed in a space defined by the sealing member between the substrates 1 and 2. The anti-ferroelectric liquid crystal 7 has a smectic layer structure formed by a plurality of smectic layers laminated each other and has a stability in molecular orientation. The liquid crystal molecular orientation is varied in accordance with an electric field.
Liquid crystal molecules of the anti-ferroelectric liquid crystal 7 are orientated at a certain tilt angle to a normal line of the smectic layer structure. As is shown in FIGS. 5A to 5C, there are three stable states of the molecular orientation.
FIG. 5A shows the=first stable state in which a strong electric field is applied in one direction to the liquid crystal layer. In this state, a spontaneous polarization of each of the liquid crystal molecules acts the electric field, so that all liquid crystal molecules are uniformly orientated in one direction at a tilt angle .theta. to the normal line L of the smectic layer structure.
FIG. 5C shows the second stable state in which a strong electric field is applied in the opposite direction to the liquid crystal layer. In this state, the spontaneous polarization of each of the liquid crystal molecules acts the opposite-directional electric field, and the liquid crystal molecules are inverted. Thus, all liquid crystal molecules are uniformly orientated in a direction opposite to the direction in the case of the first stable state, at the tilt angle .theta. to the normal line L of the smectic layer structure.
FIG. 5B shows the third stable state in which no electric field or a weak electric field is applied. In this state, the liquid crystal molecules are orientated in alternate directions in the order of layers (i.e. in alternate directions at the same tilt angle .theta. to the normal line L of the smectic layer structure). Therefore, an average orientation direction of all of the liquid crystal molecules in the liquid crystal layer is in the normal line of the smectic layer structure.
The orientation directions of the liquid crystal molecules of the anti-ferroelectric liquid crystal 7 is restricted by the alignment films 5 and 6 of the substrates 1 and 2. Thus, in the above described third stable state, the average orientation direction of the liquid crystal molecules is in accordance with a direction determined by the aligning treatments (rubbing) of the alignment films 5 and 6. In the first and third stable states, molecular longitudinal axises of all of the liquid crystal molecules are inclined by the tilt angle .theta. to the normal line L of the smectic layer structure orientated in the third stable state.
Polarizing plates 8 and 9 are laminated on the outer surfaces of the substrates 1 and 2, respectively. The polarizing plates 8 and 9 are arranged in a cross-Nicol arrangement so that their light transmission axes intersect with each other at substantially right angles. The transmission axis of one of the polarizing plates, e.g. the lower incidence-side polarizing plate 8 in FIG. 8, is substantially parallel to the average orientation direction of the liquid crystal molecules in the third stable state.
FIG. 10 shows an equivalent circuit of the above-described LCD device. A pixel, constituted by the mutually facing intersection portion of one of the scan electrodes 3 of one substrate 1 and one of the signal electrodes 4 of the other substrate 2 and the anti-ferroelectric liquid crystal 7 interposed between the intersection portion, is equivalent to a capacitor. Thus, the equivalent circuit of the LCD device is expressed by a circuit in which each of the scan electrodes 3 and each of the signal electrodes 4 are coupled by a capacitance CLC of the pixel (hereinafter referred to as "pixel capacitance").
A display operation of the above LCD device will now be described. When no voltage or a low voltage is applied across the electrodes 3 and 4 of the substrates 1 and 2, the liquid crystal molecules are orientated in the third stable state. In the third stable state, a direction of a linear-polarized light which has passed through the incidence-side polarizing plate 8 is substantially identical to the average orientation direction of the liquid crystal molecules. Thus, the linear-polarized light emanates from the liquid crystal layer structure and is shielded by the output-side polarizing plate 9 arranged in the cross-Nicol arrangement. Accordingly, in this case, light is hardly emitted from the LCD device, and the display is set in the OFF (dark) state.
On the other hand, when an ON voltage of one-directional polarity (higher than a threshold voltage of the liquid crystal) is applied across the electrodes 3 and 4 of the substrates 1 and 2, the liquid crystal molecules are orientated in the first stable state. In this first stable state, the longitudinal axes of the liquid crystal molecules are displaced by almost the same angle as the aforementioned tilt angle with respect to the direction of the linear-polarized light which has passed through the incidence-side polarizing plate 8. Thus, the linear-polarized light made incident on the liquid crystal layer is converted to an elliptic-polarized light by a birefringence effect of the liquid crystal layer, and a component of the elliptic-polarized light, which travels in the light transmission direction of the output-side polarizing plate 9, emanates from the liquid crystal device. Accordingly, the display is set in the ON (light) state.
The above description is also applicable to a case where an ON voltage of an opposite-directional polarity is applied across the electrodes 3 and 4 of substrates 1 and 2. In this case, the liquid crystal molecules are orientated in the second stable state. In the second stable state, too, the linear-polarized light made incident on the liquid crystal layer structure is converted to the elliptic-polarized light by the birefringence effect of the liquid crystal layer, and the component of the elliptic-polarized light, which travels in the transmission direction of the output-side polarizing plate 9, emanates from the liquid crystal device. Accordingly, the display is set in the ON (light) state.
FIG. 11 illustrates a typical relationship between the voltage v applied across the electrodes 3 and 4 and a light transmittance TO in the above-described anti-ferroelectric LCD device. When the applied voltage v is close to 0 V, the light transmittance TO is low, and the LCD device is in the OFF state. When the applied voltage T is increased beyond a threshold value, the light transmittance TO increases and the liquid crystal device is set in the ON state. When the applied voltage V is decreased from the value in this state and reaches below a certain threshold value which is lower than the threshold value of the ON state, the light transmittance TO decreases and the liquid crystal device becomes to the OFF state. This applies to a case where the applied voltage V is varied to the (-) side. In this case, too, when the absolute value of the applied voltage rises beyond a certain threshold value, the LCD device is set in the ON state. When the absolute value of the applied voltage is decreased from the value in this state and is reached at a certain threshold value below the threshold value relating to the ON state, the LCD device is returned to the OFF state.
Accordingly, images can be displayed on the LCD device in the following manner: voltage values +VB and -VB (FIG. 11), which are almost intermediate values between the voltage value at which the LCD device is set in the ON state and the voltage value at which the display device is returned to the OFF state, are employed as reference voltage values, and drive voltages obtained by superposing image data signals on the reference voltages +VB and -VB are applied across the electrodes 3 and 4.
FIG. 12 shows a relationship between a waveform of a drive voltage VLC, applied across the electrodes 3 and 4 of one of the pixels of the LCD device and a light transmittance T of the pixel. In FIG. 12, "TS" denotes a pixel selection time period. The time period "TS" is obtained by equally dividing one frame "TF" associated with one screen by the number of rows of pixels (i.e. the number of scan electrodes).
If a voltage much higher than the reference voltage VB is applied across the electrodes 3 and 4 of the selected pixel in the first selection time period "TS", the liquid crystal molecules are orientated in the aforementioned first stable state. Thus, the pixel is set in the ON (light) state. In a non-selection time period following the selection time period "TS", the potential of the scan electrode 3 of this pixel becomes a non-selection potential, but a data signal for driving a pixel in another selected row but in the same column as that of the pixel in the non-selection time period is applied to the signal electrode 4. Thus, a voltage of a waveform corresponding to the data signal is applied to the pixel in the non-selection time period. However, since the potential of the scan electrode 3 of the pixel in the non-selection time period is a non-selection potential during the non-selection time period, the voltage applied to the pixel during the non-selection time period is in the vicinity of +VB and a high voltage is not applied. Accordingly, the first stable state of the liquid crystal molecule orientation does not change to another stable state, although the light transmittance varies slightly owing to slight motion of liquid crystal molecules.
Suppose that a voltage ((-) voltage), whose polarity is opposite to that of the voltage in the previous selection time period, has been applied across the electrodes 3 and 4 of the selected pixel in the next selection time period "TS". In this case, the orientation state of the liquid crystal molecules is changed from the first stable state to another stable state. When the absolute value of the applied voltage is smaller than the reference voltage VB, as shown in FIG. 12, the orientation state of the liquid crystal molecules is not inverted to the second stable state but is set in the third stable state. Thus, the pixel is set in the OFF (dark) state. In the non-selection time period, as stated above, a voltage of a waveform corresponding to the data signal for driving a pixel of another row is applied. However, since the voltage applied during the non-selection time period is in the vicinity of -VB and a high voltage is not applied. Accordingly, the third stable state of the liquid crystal molecule orientation does not change to another stable state, although the light transmittance varies slightly owing to slight motion of liquid crystal molecules.
In addition, when a (-) voltage whose absolute value is much higher than the reference voltage VB is applied across the electrodes 3 and 4 of the selected pixel, the liquid crystal molecules are orientated in the second stable state and the pixel is set in the ON (light) state. Further, when a (+) voltage whose absolute value is smaller than the reference voltage VB is applied, the liquid crystal molecules are orientated in the third stable state and the pixel is set in the OFF (dark) state.
In this anti-ferroelectric LCD device, the liquid crystal molecules retain one of the first, second and third stable states until an electric field to change the orientation state is applied. Thus, the display with higher contrast can be obtained, as compared to generally employed TN mode or STN mode liquid display devices. Moreover, a time-sharing driving with high duty may be possible, realization of the LCD device with a high precision and a large screen can be expected.
The currently known anti-ferroelectric liquid crystals, however, have a problem that a response time to electric field is long. In addition, since the conventional anti-ferroelectric LCD device is of the simple matrix type, as mentioned above, a frame frequency for writing one screen cannot be increased.
The reason for this is that, in the simple matrix type LCD device, the electric field to change the orientation state of the liquid crystal molecules is applied to the liquid crystal during only the selection time period of each pixel, and therefore to set the pixel in the ON state or OFF state must be completed within the pixel selection time period.
More specifically, in the simple matrix type LCD device, the electric field to change the liquid crystal molecule orientation state acts on the liquid crystal during only the selection time period. Thus, unless the change of the liquid crystal molecule orientation state is completed within the selection time period, the setting of the pixel in the ON or OFF state becomes defective and image display cannot be effected.
Under these circumstances, in the conventional anti-ferroelectric LCD device, the selection time period of the pixels of each row needs to be set to a time period necessary for the change of the liquid crystal molecular orientation state. However, the response time to electric field is long in the anti-ferroelectric liquid crystal, and if the selection time period is set according to the response time of the anti-ferroelectric liquid crystal, the frame frequency becomes less than a practical value and high duty time sharing driving is not achieved.
Furthermore, in the conventional anti-ferroelectric LCD device wherein the aligning treatment directions of the alignment films of substrates 1 and 2 of the liquid crystal cell are parallel to each other, a light leak amount (i.e., amount of light not absorbed by the emission-side polarizing plate and emanating therefrom) in the OFF state is large. Consequently, the light/dark contrast is not good.